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Seacock backing plates -- flush mounted fitting

I have 7 thru hulls. I am replacing all with new fittings and seacocks. I will glass over one opening at the area of the head but will use the rest of them.

Problem is that only one of the 6 has a typical flat mushroom fitting so that the inside of the boat is flat and so will the backing plate. Making that one is not a problem. However, the other thru hull fittings (5) have flush thru hulls fittings on the outside of the hull so the inside of the boat is in-setted to accomodate the flush thru hull fitting. So how do I make the backing plate for the rest of the seacocks backing plates when there is a bump inside the boat to allow for the lip of the thru hull to be flush on the outside of the boat? I have look over Youtube which has some instruction and other online instruction but none addresses flush seated thru hull fittings having a rise or mound inside the boat the shape of the flush fitting.

Any suggestions on making a backing plate. Should I do it out of wood and ground out a hump to fit over the hull shape. Hope all this makes sense.


lee nicholas
Two ways use 6 inch circles of fiberglass say 5 or six and layer over the thru hull spot . glass resin glass resin etc or make one out of plywood I like marine and glass two layer over that .
in my 384 i had all the thru hull removed , they drilled 3 inch holes from outside to inside cut off the thru hull . re glassed the hole and recut it with a glass pad layer inside like 7-9 inches around i went with Marlon thru hulls and valves as my brass were all frozen. Now its all labeled and every thing works.
Lee, I went with all brass/bronze thru hulls and seacocks because of the flush mount thru hulls fittings seemed hard to get in Marlon. I read pros and cons on each and just defaulted on brass. If I understand, you are suggesting two plates, one larger in the center and then another on top?


lee nicholas
Nope just one plate you can either make it out of multi layers of glass or use plywood. The advantage if all glass is never any rot ! Make mat circles cut a hole for the bronze thru hull , coat the bronze with veg oil so resin and glass will not stick and do layers like 6-8 say 6 inch circles to build up the area. what your doing is building up a strong pad to beef up the hull against the twisting of the seacock , cause the bronze old sea cocks got real hard to turn as they age putting undue stress in the hull. The pad spread out the load. Hope this helps


Richard Dowe
Morning John, I have replaced just one unit and was surprised to find the original unit beveled into the hull. I used the original threaded piece and bought a Greco valve. I made a slab of fiberglass 1/2 thick and cut two 6 inch circles. One piece I cut a second hole that was larger than the bevel, the second I cut the size of the thru - hull. I taped the threads on the thru-hull and waxed it. On the second circle I drilled and tapped three holes for the Greco valve. I than made a mix of resin, cut fiberglass, and thickening agent. I wax the bottom of the valve and temporarily bolted the valve on the circle. I inserted the thru-hull and held it in place with a 2x4 from the ground. I spread the compound on the hull and pressed the first circle into the compound. Than I filled the void with more of the fiberglass and placed the second circle with the valve on it and screwed it down over the thru-hull. After it cured I removed the valve and thru-hull and removed the wax applied 4200 on the thru-hull and valve and screwed it together.


Terence Thatcher
In use glass reinforced plastic for backing plates. It had a letter and number designator, but I forget it. Sorry. If you are concerned about the bump, cant you just seat the backing plate in epoxy thick enough to fill the gap? By the way, I did not through bolt the sea cocks through the hull. I used the alternate method suggested by Compass Marine web site, with my bolts sticking out from the backing plate, with the head countersunk on the side of the backing plate that touches the hull interior surface. Sorry for the bold font,. Not sure what happened.
Thanks all. Rick and Terry, I was thinking of making the backing plate the same way you described. 1/4 inch from inside of hull almost gets me level with the top of the hump. So, I thought of one backing plate with a larger hole to clear the entire bump and the second to have a hole the size of the thru hull fitting sitting on top of the hump and supported by the lower plate. The top plate would have the three holes for the bolts insetted. Epoxy all at one time using Six10 epoxy from west marine. I would use The G10 fiberglass plates. The epoxy Six10 would fill in any voids. My biggest concern is not having the plate off of level so that the thru hull lip would not sit inside the inset. I would try first with a dry fit. I'm I missing something something with this method?


Terence Thatcher
You can coat all the through hull parts with a good wax and use it to install and level the backing plate. Once everything mis set up, remove the fittings, pout on your caulking compound Andy install. With eno8ugh wax the epoxy will not stick to the parts.


Jeff Lovett
I suggest going with all mushroom style thru hull fittings. Either relocate the thru hull or completely fill in the recess and re-drill the hole. I replaced all of Pilgrim's original thru hulls and seacocks with Marelon hardware. The only recessed fitting I kept was the head sink drain. All other thru hulls were relocated and I used the mushroom style hardware. Wish I had converted the head sink drain to mushroom style fitting as well. Grinding out and filling the original thru hull recesses I discovered that the hull material in these area is very thin relative to the rest of the hull. Images and notes from the thru-hull replacement are available here... Re-Plumbing Pilgrim Photo Album

To more directly address your question about creating a backing plate for the recessed fittings.... I used two layers of 3/8" thick fiberglass reinforced panel (FRP or G10) In the layer closest the hull I cut a inside circle large enough to encompass the recess. I then filled any voids with milled fiber thickened epoxy. After this cured I used a machine screw to center and clamp the top piece of FRP. Details on the fabrication are available on my website. Here is a link to the post.... Installing Seacock Pad for Flush Mounted Thru-Hull

Here is an image of using a machine screw to position and clamp the top FRP panel in place.

The large pad pictured above is new location for offshore head discharge. Visible above that pad is the area where I filled in two original recessed thru-hull fittings.

Here is a picture of the completed install.
Jeff, as the undisputed fiberglass Czar, I appreciate your response. I guess recessed thru hulls were/are used to avoid tuberlance when the boat slides through the water, but I wonder if the benefit justifies the additional work. In addition to customizing the backing plates, I have 5200 to dig out around the outside recessed openings when after I am done, will explain why sailors have the reputation for using colorful language. I would like to do what you did but I think that's little bit above my skill level. I will glass over one thru hull opening and have my sink and shower drain into one seacock. I'll order the pads and use two, one to go over the hump and the other for the fitting opening. I have all the bronze now, but I need to order the backing plates and will study your blog and the responses to this quiry for which I appreaciated all who have given their input.
Here are a few pictures of my backing plate build and the placement of them under galley sink before installing the seacock. Because of the flush mount thruhull inset, I had to put two plates for each seacock valve. One to get a flat surface for the seacock, then the second plate is epoxyed on top with the bolts already in the second plate that I bolted the seacock on. What you see is the thru hull being put in backwards just to temporarily hold both plates in place while the thickened epoxy cured. The next day, I then pulled out the thruhull, fastened the seacock with 5200 and then from outside, screwed in the thruhull with 5200 on the inside lip of the thruhull for a seal. (I followed a youtube instruction that was nicely done for installing them). I will post other pictures with the seacocks all installed once I am done. I have completed installing 4 seacocks so far, one more to go. I have glassed over one opening in the head that I will not be using that had a 1 1/4 inch thruhull.



Mark Pearson
Staff member
1) Fresh water in, 2) Sink drain out, 3) Head drain out.

Here is a snippet from our antique Owner's Manual:
As noted by the posts below, it seems most people probably have 2 each 1-1/4" and 1 each 3/4". The owner's manual snippet from above is probably dated. I think in our case the functioning of each thru hull is accurate as described in the manual.
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James M. Cleary
John Gonzalez
When you poured the backing plates, did you use fiberglass mat, roving or cloth? And what kind of resin was used?

I did JIm. I can't remember the size. But I alternated matt with cloth. You have to do that otherwise it is all expoy and not strong. To make the backing plates for the seacocks requires drilling holes for the bolts so matting is necessary to hold the bolt and seacock without cracking.
I did JIm. I can't remember the size. But I alternated matt with cloth. You have to do that otherwise it is all expoy and not strong. To make the backing plates for the seacocks requires drilling holes for the bolts so matting is necessary to hold the bolt and seacock without cracking.
John, can you check your conversations, I sent you a private message. Brian Zinser


I just did my last thru hull this year. I have been putting it off for a long time because it's too hard to get to. (Or I'm to fat) so when I pulled the boat this year for bottom paint I did it. Grinder with flap wheel, west system with silica,fiberglass board.